In U.S. Pat. No. 5,793,750, the contents of which are hereby incorporated by reference, a communication system between two microprocessor-based protective relays for an electric power system is disclosed. Each of the two relays in that system has both transmit and receive modules, for directly transmitting output status bits indicative of the result of selected protective functions of one relay from that one relay to the other, and vice versa.
The output status indications, also referred to as output function information, in the form of bits, are used to identify the existence and location of a fault on the power line portion served by the two relays. One or both of the relays might initiate a circuit breaker trip action on the basis of the exchange of such information. The output status indications (bits) are the result of processing functions in one of the relays involving the voltages and/or currents on the power line. The output status indications are used for various relay and relay operations, including permissive overreaching transfer trip (POTT) actions, permissive under-reaching transfer trip (PUTT) actions, directional comparison unblocking (DCUB) and direct transfer trip (DTT) actions. Other relay-to-relay operations are possible using particular output status indications.
The advantage of the communication system described in the '750 application is that it is fast and secure. Protective relays typically accomplish their monitoring functions each 1/16 of a power system cycle or approximately every millisecond. The '750 communication system provides the results of these functions in one relay to the other relay. The information is transmitted directly over a communications link from an originating relay which may or may not trip its associated circuit breaker, based on its operational results, to the other relay, which uses the transmitted information, in the form of digital bits, to perform its own on-going calculations, producing a trip signal when appropriate. The communication between the two relays is bi-directional, allowing the two relays to exchange information concerning the results of their own calculations both quickly and securely, with a minimum amount of expense.
In the '750 application, the output function capability is eight bits. In many cases, however, eight channels are not necessary. Two or three bits are usually sufficient to accomplish the desired relay-to-relay protective scheme. A substantial number of bits therefore may go unused. The present invention makes use of those otherwise unused bits. It forms a serial data stream or channel from each unused bit, and utilizes those serial data channels to significantly increase the communication of information between the two relays. This invention may utilize none, some or all of the eight channels to transfer output status indications. If less than all eight channels are used for output status indications, the otherwise unused channels may be used to transfer other selected information.